The present invention relates to polycarbonate-polyester compositions. More particularly, the present invention relates to improved polyester-polycarbonate compositions stabilized against ester-carbonate interchange between the polyester and the polycarbonate.
Polyester-polycarbonate compositions are widely used in industry. However, a disadvantage of these compositions is their tendency to undergo ester-carbonate interchange, wherein ester linkages in both the polycarbonate and the polyester are broken and replaced by alkylene carbonate and aryl carboxylate bonds. The result is degradation of the physical properties of the polymers due to hybridization of the molecular linkages therein. This in turn leads to variability in the final fabricated article.
It is believed that the ester-carbonate interchange in polyester-polycarbonate compositions is promoted by metallic catalyst residues present in the polyester. These residues are left over from the polymerization reaction forming the polyester, wherein certain metal compounds are used as polymerization catalysts. It appears, however, that these metal compounds also catalyze the transesterification reaction between the polycarbonate and the polyester.
It would be desirable to provide a compound which deactivates the metallic catalyst residues present in the polycarbonate/polyester compositions. The resulting polyester-polycarbonate compositions would be improved in that they would have a decreased tendency to undergo ester-carbonate interchange and therefore would be stable against such interchange.
Although methods for deactivating metallic catalyst residues in polyester-polycarbonate compositions are known in the art, as described hereinbelow, it is continually desirable to provide alternative methods for deactivating such residues.
It is known in the art that certain phosphorous-containing compounds are useful in deactivating metallic catalyst residues. Reference is made, for example, to U.S. Pat. Nos. 4,532,290 (Jaquiss et al.) and 4,401,804 (Wooten et al.). Examples of such phosphorous-containing compounds include phosphoric acid, certain organic phosphorous compounds such as distearyl pentaerythritol diphosphate, mono- or dihydrogen phosphonate or mono-, di-, or trihydrogen phosphate compounds, or di- and triester phosphonate compounds, phosphite compounds, and certain inorganic phosphorous compounds, such as monosodium phosphate and monopotassium phosphate, alone or in combination.
Certain of these phosphorous-containing compounds, e.g., phosphoric acid, have the drawback of reacting with and neutralizing acid-sensitive pigments, dyes or other coloring agents added to the polycarbonate/polyester compositions, thereby leaving the composition with an undesirable color.
It would be desirable to provide a compound which deactivates the metallic catalyst residues present in the polycarbonate/polyester compositions without reacting with and neutralizing acid-sensitive pigments or dyes added to the compositions.
The present invention is based on the discovery that certain silyl phosphates will effectively deactivate metallic catalyst residues in a polycarbonate-polyester composition, without neutralizing acid-sensitive pigments or dyes added to the polymer composition. The silyl phosphates used in this invention will inhibit the ester-carbonate interchange promoted between the polyester and the polycarbonate by the metallic catalyst residues.
None of the references cited above teach the use of a silyl phosphate for inhibiting the ester-carbonate interchange in a polyester-polycarbonate composition.